Wave guide mode transformer



Sept. 27, 1955 w PRESTQN 2,719,271

WAVE GUIDE MODE TRANSFORMER Filed Aug. 2, 1945 FIGJ 4&

INVENTOR WILLIAM M, PRESTON- ATTORNEY United States Patent 0 WAVE GUIDEMODE TRANSFORMER William M. Preston, Lincoln, Mass., assignor, by mesneassignments, to the United States of America as represented by theSecretary of the Navy Application August 2, 1945, Serial No. 608,590

9 Claims. (Cl. 333-21) This invention relates to the guided transmissionof electric waves, and more particularly to apparatus for transformingthe oscillation of electric waves from one mode to another as said wavesare interchanged between cylindrical wave guides and other forms oftransmission lines.

The subject matter of my present invention is disclosed in part in theapplication of Shepard Roberts, S. N. 510,990, filed November 19, 1943,for Rotatable Joints for Radio Wave Guide Systems. The rotatable jointstherein described illustrate a valuable use for the mode transformers ofmy present invention.

It is frequently desirable in the guided transmission of electric wavesto employ a cylindrical wave guide carrying waves in a mode ofoscillation having symmetry about the longitudinal axis of the guide,such as the TMor mode. In radio echo detection systems, for example, itis usual to guide electric waves to a rotatable antenna structure from agenerator of the waves, and from such a structure to a receiver for thesaid waves. The use of the TM01 mode of oscillation in a cylindricalwave guide permits the advantageous use of a guide made of two coaxiallyaligned abutting sections, mutually relatively rotatable on their commonaxis, at the rotatable antenna structure. Rotation of one section of thewave guide relative to the other section does not disturb the TM01 wavestherein and consequently a limitation upon the rotatability of the saidstructure is removed.

For various other reasons however, it is not desirable to use the TM01mode of oscillation to transmit electric waves from one point toanother. Among these reasons is the fact that the TM01 mode ofoscillation is not the lowest mode for cylindrical wave guides. Such aguide having a diameter sufliciently great to carry energy in the TM01mode is also large enough to carry energy in the TE11 mode, which doesnot have axial symmetry. The presence of more than one mode ofoscillation in a wave guide results in a waste of power, since the powerin the added mode is not used. It has been found to be more desirable touse a rectangular wave guide carrying energy in the lowest mode ofoscillation, or a coaxial line as the main transmission line, and toprovide a rotatable joint in the line where necessary.

Perhaps the simplest form of rotatable joint is that which makes use oftwo relatively short sections of cylindrical wave guide, excited andcoaxially abutted as hereinabove set forth. The main transmission lineis connected into each end of this joint, and energy is transferredthrough the joint in either direction from and to the said maintransmission line. Waves being carried in a desired mode in the maintransmission line must then be transformed into the TM01 mode uponentering a cylindrical wave guide section, and back to the transmissionline mode upon leaving the cylindrical guide. Thus each end of arotatable joint that is used in such an installation must be a modetransformer for the electric waves being carried.

In the past it has been usual to employ suitable antennas and supportswithin transmission lines at points of junction with rotatable joints toeffect mode transformations. Such structural arrangements are difficultto construct, especially at the higher frequencies where wave lengthsare short and machining tolerances consequently very close. Thesestructures also cause numerous electrical difliculties. In the casewhere a rectangular wave guide is the main transmission line, theinsertion of antenna structures is particularly objectionable, for itrequires additional structure within the wave guides that makes thejunction relatively very complicated.

In my invention I provide mode transformers having a rectangular waveguide coupled directly into the side of a cylindrical wave guide,without the use of coupling antennas or supports within either waveguide. The junction between the two wave guides is solid and has nomoving parts. One end of the cylindrical wave guide is provided with anelectrical closure, and this end forms a short circuited stub arrangedin the diflerent embodiments of my invention in novel ways to inhibitthe formation of the unwanted TE11 mode of oscillation in thecylindrical wave guide and to facilitate the generation of the desiredTM01 mode.

It is an object of my invention to provide a mode transformer for Waveguide systems that will transform Waves of energy in the lowest mode ofoscillation in a rectangular wave guide into waves in the TMor mode ofoscillation in a cylindrical wave guide without the use of couplingantennas or the like.

It is a further object of my invention to provide such a transformerthat will substantially inhibit the formation of waves in the TE11 modein the cylindrical wave guide while simultaneously facilitating thegeneration of waves in the TM01 mode of oscillation therein.

It is a still further object of my invention to provide such atransformer that will be operable in either direction to transform wavesin the desired mode in one wave guide into waves in the desired mode inanother wave guide thereunto connected.

It is another object of my invention to provide such a mode transformerthat will be simple and easy to construct, will have no moving parts,and will require no unusually close machining tolerances.

Other objects and features of my present invention will become apparentupon a careful consideration of the following detailed description whentaken together with the accompanying drawings, the figures of whichillustrate typical embodiments of the invention.

Fig. 1 illustrates in cross section a mode transformer constructed inaccordance with my invention;

Fig. 2 is a cross section taken along line IIII of Fig. 1;

Fig. 3 is a cross section of another mode transformer constructed inaccordance with the teachings of my invention; and

Fig. 4 is a cross section taken along line IV-IV of Fig. 3.

In Figs. 1 and 2 a rectangular wave guide 1, having width a andthickness b, is joined at one end thereof to a cylindrical Wave guide 2near one end thereof. The longitudinal axes of the two wave guides 1 and2 are preferably mutually perpendicular, although variations in theangle between said axes is permissible. The said near end of thecylindrical wave guide 2 is closed by a metallic end closure 3. Thejunctions of the two wave guides 1 and 2 and of the cylindrical waveguide 2 with its end closure 3 are preferably made with solder, althoughany other means of providing mechanical fixity and electrical contact atsaid junctions is suitable. Diaphragms 4, made of relatively thinmetallic plates, preferably rectangular in shape, are inserted one ineach narrow wall of the rectangular wave guide 1, in a plane preferablysubstantially perpendicular to the longitudinal axis of that waveguide 1. One diaphragm 4 may be used alone, if desired, but the use oftwo is preferred. When two diaphragms 4 are used, they are preferablyco-planar.

The diaphragms 4 are provided for the purpose of effecting an impedancematch between the two wave guides 1 and 2. The distance 5 between thetwo diaphragms 4 of Fig. 2 and the distance 6 from the plane of thediaphragms 4 to the inner wall 7 of the cylindrical wave guide 2 areboth chosen to provide the desired impedance match. These dimensions 5and 6 are commonly arrived at by experiment and are susceptible ofvarious satisfactory combinations. One satisfactory set of dimensions isdisclosed hereinbelow for similar matching diaphragms used in theapparatus of Figs. 3 and 4.

The diameter 8 of the cylindrical wave guide 2 should be large enough.to permit the passage of energy in the TM01 mode of oscillation, but notlarge enough to permit the passage of higher modes. The cut-off diameterfor the TM01 mode of oscillation is given by the expression where d isthe diameter 8 of the cylindrical wave guide 2, and is the free-spacewave length of energy at the frequency being transmitted. Since thecut-off diameter for the TE11 mode of oscillation is equal to 058k, acylindrical wave guide 2 of large enough diameter to carry the TM01 modeof oscillation will also sustain oscillations in the TEn mode. The nexthighest mode of oscillations for cylindrical wave guides is the TE21mode, for which the cut-off diameter is equal to 097k. Therefore, inorder not to permit the passage of the TE21 mode of oscillation, thediameter 8 of the cylindrical wave guide 2 should be smaller than O.97Hence the diameter 8 will preferably be determined at a value between0.76% and 0.97%. In practice, a diameter 8 of a magnitude approaching0.97% is preferably used, as smaller diameters 8 cause relativelygreater attenuation of the signal being carried by the wave guide 2 thando larger diameters.

The junction point of the two wave guides 1 and 2 may be considered tobe that point at which the two longitudinal axes of said wave guidescross, although it is to be understood that energy is interchangedbetween the two wave guides 1 and 2 in a region about such a point. Thedistance 9 from such a point to the end closure 3 may be regarded as theclosed stub length, and should be chosen and adjusted at a value whichwill inhibit the propagation of waves in the TEu mode. The stub length 9is accordingly preferably substantially equal to an odd number ofquarter-wave lengths in the cylindrical wave guide 2 of energy in theTEu mode of oscillation.

The rectangular wave guide 1 is of an ordinary kind in which the width(2 and thickness b are of magnitudes that will permit the said waveguide 1 to carry energy in the lowest or TEIO mode only. In the TEiomode for rectangular wave guides, the electric vector is in a directiontransverse to the longitudinal axis of the wave guide and perpendicularto the wide walls thereof. Thus, in Figs. 1 and 2, the aforesaidelectric vector will be substantially parallel to the line indicatingthe narrow, or b dimension, and perpendicular to the line indicating thewide, or a dimension.

The apparatus of Figs. 1 and 2 operates in a relatively simple manner.For the sake of explanation it will be assumed that energy is beingtransferred from the rectangular wave guide 1 to the cylindrical waveguide 2, although it should be borne in mind that energy may be passedin either direction in this apparatus. Accordingly, energy in the TEiomode for rectangular wave guides enters the cylindrical wave guide 2from the rectangular wave guide 1. In the region of entry, thisaforementioned TEm energy may generate waves of many modes forcylindrical wave guides. However, since the diameter 8 has a magnitudethat is less than 0.97%, as hereinabove explained, those modes higherthan the TM01 and.

the TE11 modes will quickly be attenuated to insignificant intensities,and may be regarded as absent for all practical purposes.

The newly generated waves in the TM01 and TEu modes will proceed alongthe cylindrical wave guide 2 away from the aforesaid region. Those wavesthat enter the closed stub and encounter the end closure 3 will bereflected back toward the aforesaid region, where they will encounterother newly generated waves. Since the length 9 of the stub issubstantially equal to an odd number of quarter wave lengths in the waveguide 2 for the TE11 mode, reflected TE11 mode waves will arrive at theaforesaid region substantially degrees out of phase with newly generatedwaves present in said region, and the said TEu mode waves willsubstantially mutually cancel each other. As a consequence, there willbe a relatively great attenuation of waves in the TEu mode. From anotheraspect, it may be said that the mode transformer of Figs. 1 and 2presents a relatively high input impedance to the TE1 mode.

As is known to those skilled in the art, the wave length in the waveguide will be different for waves in different modes of oscillation atthe same frequency. In the illustrative mode transformer of Figs. 1 and2, air filled wave guides are used. Thus, in the cylindrical wave guide2, a wave in the TMoi mode is longer than a wave in the T1311 mode atthe same frequency. Therefore, a depth 9 for the closed stub end of thecylindrical wave guide 2 that will result in substantial cancellation ofthe TEll mode waves will not cause the cancellation of waves in the TM01mode.

In order that the most favorable impedance match may be had between thetwo wave guides 1 and 2 for the generation of waves in the TM01 mode,the diaphragms 4 are installed in the rectangular wave guide 1. Thesediaphragms 4 introduce an inductive shunt susceptance into the modetransformer system. As the spacing 5 between the diaphragms 4 isdecreased, this susceptance is increased. This spacing 5 is preferablychosen for the most favorable impedance match for the generation andpropagation of waves in the TM01 mode.

When conversely it is sought to transform waves in the TM01 mode in thecylindrical wave guide 2 into waves in the TEio mode in the rectangularwave guide 1, the above described action will occur in reverse. Due tothe aforementioned favorable impedance conditions, waves in the TM01mode will be present in relatively great strength at the region ofjunction of the transverse guides 1 and 2. These waves in the TMoi modein the cylindrical wave guide 2 may generate waves in many modes in therectangular wave guide 1. However, the dimensions a and b are such thatas hereinabove stated, only waves in the TEm mode for rectangular waveguides will be propagated in the wave guide 1. All other modes will besubstantially attenuated to insignificant values of intensity. Thus, itis seen that the apparatus of Figs. 1 and 2 may be used to transformelectric waves from one mode in a rectangular wave guide 1 into anothermode in a cylindrical wave guide 2, or vice versa.

It is possible to choose a value of the diameter 8 for the cylindricalwave guide 2 that will permit the stub end of said wave guide to effectsubstantially complete cancellation of the TE11 mode waves andsimultaneously bring about substantially complete reenforcement to theTM01 mode waves. As hereinabove mentioned, the wave length in the waveguide 2 of waves at a given frequency is different for different modes.In general, for an airfilled wave guide, the wave length in the 'guideof energy being carried therein is given by the relation;

where:

X =the length in the waveguide of a wave offthe en'erg y beingcarried; f7\=the free-space wave length of that energy; and Ac=the free-spacecut-off wave length for the particular mode in which the energy is beingcarried." Since the cut-off wave length in a wave guide differs fordifferent modes of propagation, the wave length in the guide will alsodiffer for different modes of propagation. In a cylindrical wave guide,the .cut-off wave length for the TMo1 mode is given by the relation: H

i. 9 =1.31'd where =the free-space cut-off wave length for the TMo1 modeand d is the diameter of the cylindrical of energy at the operativefrequency in the TMM mode, and I v TEu zthe wave length in thecylindrical wave guide of similar energy in the TE11 mode. 7 1

If now the diameter d be given such a value that TEn g TMm then itbecomes apparent by simple algebraic process that TEH L TMM From thislast relation it is evident that a section of cylindrical wave guidehaving this special value of diameter d may at one and the same time bethree-quarters of a wave length long for the TE11 mode and one-half awave length long for the TM01 mode, both wave lengths being in-the-guidewave lengths. Thus, in the apparatus of Figs. 1 and 2, if the diameter 8be giventhe special magnitude that will result in the relation and, ifthe stub length 9 be made to be substantially equal to three-quarters ofthe length in the wave guide 2 of a wave in the TE11 mode, the stub endof the waveguide 2 will simultaneously substantially wholly cancel theTE11 mode and substantially wholly reenforce the TMM mode ofoscillation.

The diameter 8 of the cylindrical wave guide 2 that will render may befound by simple algebra from the relation! Thus the diameter 8 of thecylindrical wave guide 2 should be equal to 0.884 times the free spacewave length of the energy being carried therein to render Thus, in theapparatus of Figs. 1 and 2, when the diameter 8 of the cylindrical waveguide 2 is chosen to be- 0.884 and the stub length 9 is equal tothree-quarters of a TE11 mode wave length in the wave guide, the saidstub length 9 will simultaneously be equal to one-half of a TM01 modewave length in the wave guide 2. By the ordinary principles of wavepropagation, the stub end of thewave guide 2 will then simultaneouslyefiect substantially complete cancellation of the TE11 mode waves, andsubstantially complete reenforcement of the TM01 mode waves. Inasmuch asthe stub. length 9 has substantially an ideal length for the TMo1 mode,the amount of susceptance required to be provided by the diaphragms 4will be only very slight.

The diameter 0.8841 is not the largest that may be used for the purposesof the hereindisclosed mode transformers. As hereinabove set forth,diameters 8 approaching the value 0.97% are desirable. A diameter 8 ofthe value 0.8841 may result in undesirable attenuation of thetransmitted signal. Accordingly, another value for the diameter 8 hasbeen investigated, and found useful. When the diameter 8 is of a valueequal to 0.94A, the following relation is true:

neously has a value equal to 0.56 of a TMOl wave length in the said waveguide. The value is substantially close to the value one half ATMOA sothat there is still a relatively good impedance match for the TM01 modeof oscillation. Some additional shunt susceptance may be provided by thediaphragms 4 if desired to effect the best possible impedance match.

In Figs. 3 and 4 there is illustrated a modification of my inventionthat combines the desirable features of smaller and larger diameters ofthe cylindrical wave guide portion of the mode transformer. Therectangular wave guide 1 is at one end inserted into the side of acylindrical wave guide 10 near an end thereof. The said end is closed byan end closure 3 substantially identical to the end closure 3 of Fig. l.The internal diameter 11 of the closed stub portion of the cylindricalwave guide 10 is smaller than the internal diameterv 12 of the remainderof the said cylindrical wave guide. The portion of smaller diameter 11does not extend the full distance 13 to the center line of therectangular wave guide 1, but extends a lesser distance 14 to the innersurface 15 of the upper wide wall 16 of the rectangular wave guide 1.The wave guide 1 is flush at the ends of its upper and lower walls 16and 17 with the adjacent inner surfaces 18 and 19 respectively of thecylindrical wave guide 10. Other parts bearing the same referencecharacters as corresponding parts in the apparatus of Figs. 1 and 2 aresubstantially identical to said corresponding parts.

With the apparatus of Figs. 3 and 4, it is possible to choose a valuefor the diameter 11 of the cylindrical stub in the neighborhood of O.884and at the same time have the diameter 12 of the remainder of thecylindrical wave fixed at an advantageous value, such as 0.947\- Apreferred value for the length 14 of the cylindrical stub may bedetermined experimentally. I have determined many sets of preferredvalues for the diameters 11 and 12 and the stub length 14. One suchpreferred set of values is herein presented.

The stub diameter 11=0.845 The stub length 14 =0.835 The larger diametermay remain 0.94)\

I have further determined a set of satisfactory values for the opening 5in the inductive diaphragms 4 and their spacing 6 from the lower innerwall 19 of the cylindrical wave guide 10. These values are herepresented:

The opening 5=0;525)\ The spacing 6=O.144)\ The hereinabove statedvalues of various dimensions, are not to be regarded as the onlyworkable values, but merely as an example of one set of preferred valuesthat yield successful performance.

A rectangular wave' guide 1 having the dimensions:

functions well in either of the mode transformers illustrated in thedrawings. 7

Although I have shown and described only certain specific embodiments ofmy invention, I am fully aware of the many modifications possiblethereof. Therefore this invention is not to be limited except insofar asis necessitated by the prior art and the spirit of the appended claims.

I claim:

1. A wave transformer for electric waves comprising a transmission lineadapted to carry electric waves, a hollow cylindrical Wave guide adaptedto carry electric waves in the TM01 and TEu modes of oscillation, thediameter of said wave guide being such that a wave in said TMoi mode issubstantially three halves the length of a wave in said TE11 mode, andan end closure in one end of said wave guide, said transmission linebeing at one end joined and electrically coupled to said wave guidethroughv a hole in the side thereof near said end closure, the distancemeasured along the longitudinal axis of said wave guide from said endclosure to the mid point of said hole projected perpendicularly to saidaxis being simultaneously substantially equal to three quarters of awave length in said TE11 mode and one half a wave length in said TMoimode.

2. A Wave transformer for electric waves comprising a transmission lineadapted to carry electric waves, a hollow cylindrical wave guide adaptedto carry electric waves in the TMM and TE11 modes of oscillation, and anend closure in one end of said wave guide, said transmission line beingat one end joined and electrically coupled to said wave guide through ahole in the side thereof. near said end closure, the diameter of thatportion of said wave guide between said transmission line and said endclosure being smaller than the diameter of the remainder of said waveguide, said smaller diameter and the length of said portion beingrelatively so proportional that waves in the TEM mode proceeding throughsaid wave guide from the region of said hole in a direction away fromsaid end closure are substantially wholly cancelled by like wavesreflected from said end closure, while simultaneously waves in the TMo1mode proceeding through said wave guide from the regionofsaid hole in adime-- tionaway from said end closure are substantially whollyreenforced by like waves reflected from said end closure.

3. A wave transformer for microwaves comprising a hollow rectangularwave guide adapted to carry microwaves, a cylindrical wave guide closedat one end and 7 adapted to carry microwaves in a first mode having asymmetry about the longitudinal axis thereof and in a second modenothaving such symmetry, said rectangular Wave guide and said cylindricalwave guide being electrically coupled together by an aperture in theside of said cylindrical wave guide, the location of said aperturedefining a stub length of cylindrical wave guide related to thewavelength of said second mode in said guide to suppress said secondmode and to reinforce said first mode by reflection from said closed endof said cylindrical wave guide.

4. A wave transformer for microwaves comprising a rectangular wave guideadapted to carry microwaves, a hollow cylindrical wave guide adapted tocarry microwaves in a first mode having symmetry about the longitudinalaxis thereof and in a second mode not having such symmetry, the diameterof said cylindrical wave guide being such that a wave in said first modehas a wavelength substantially three halves of the wavelength of a wavein said second mode, an end closure in one end of said cylindrical waveguide, said rectangular wave guide being at one end joined andelectrically coupled to said cylindrical wave guide through a hole inthe side thereof near said and closure, the distance measured along thelongitudinal axis of said cylindrical wave guide from said end closureto the mid point of said hole projected perpendicularly to said axisbeing substantially equal to three quarters of a wavelength of a wave insaid second mode in said cylindeical wave guide.

5. A wave transformer for microwaves comprising a rectangular wave guideadapted to carry microwaves, a hollow cylindrical wave guide adapted tocarry microwaves in a first mode of oscillation having symmetry aboutthe longitudinal axis thereof and in a second mode of oscillation nothaving such symmetry, an end closure in one end of said cylindrical waveguide, said rectangular wave guide being at oneend joined andelectrically coupled to said cylindrical wave guide through a hole inthe side thereof near said end closure, the diameter of that portion ofsaid cylindrical wave guide between said rectangular wave guide and saidend closure being smaller than the diameter of the remainder of saidcylindrical wave guide, said smaller diameter being such that a wave insaid first mode has a wavelength substantially three halves of thewavelength of a wave in said second mode, and the length of said portionbeing relatively so proportioned that Waves in said second modeproceeded through said wave guide from the region of said hole in adirection away from said end closure are substantially wholly cancelledby like waves reflected from said end closure, while simultaneouslywaves in said first mode proceeding through said cylindrical wave guidefrom the region of said hole in a direction away from, said hole aresubstantially reinforced by like waves reflected from said end closure.

6. A wave transformer for electrical waves comprising, a hollowrectangular wave guide adapted to carry electric waves, a hollowcylindrical wave guide closed at one end and adapted to carry-waves in afirst mode having symmetry about the longitudinal axis thereof and asecond mode not having such symmetry, said rectangular wave guide andsaid cylindrical wave guide being electrically coupled together by anaperture in the side of said cylindrical wave guide near said closed endthereof, said aperture being so located with respect to the closed endof said cylindrical wave guide that electric waves in said second modereflected from said closed end cancel similar waves in said second modeexisting in said cylindrical wave guide in the vicinity of said apertureand that waves in said first mode reflected from said closed endeifectively reinforce similar waves in said first mode existing in thecylindrical wave guide in the vicinity of said aperture.

7. Apparatus for transforming electromagnetic energy from a TE11 mode toa TMOI mode or vice versa comprising, in combination, a hollow sectionof rectangular wave guide, a hollow section of cylindrical wave guide,an end closure in one end of said cylindrical wave guide, saidrectangular wave guide being at one end joined and electrically coupledto said cylindrical wave guide through a hole in the side thereof nearsaid end closure, the diameter of said cylindrical wave guide beingbetween .884 and .970 of the free space wave length of theelectromagnetic energy being propagated within the apparatus and thedistance measured along the longitudinal axis of said cylindrical waveguide from said end closure to the midpoint of said hole projectedperpendicularly to said axis being substantially equal to .75 of thewave length in the cylindrical wave guide of the electromagnetic energyin the TE11 mode guide.

8. Apparatus for transforming electromagnetic energy from a TE11 mode toa TMui mode or vice versa comprising, in combination, a hollow sectionof rectangular wave guide, a hollow section of cylindrical wave guide,an end closure in one end of said cylindrical wave guide, saidrectangular wave guide being at one end joined and electrically coupledto said cylindrical wave guide through 10 an aperture in the sidethereof near said end closure, the diameter of that portion of saidcylindrical wave guide between said rectangular wave guide and said endclosure being approximately .845 of the free space wave length of theelectromagnetic energy being propagated within the apparatus, thediameter of the remainder of said cylindrical wave guide beingapproximately .940 of said wave length and the distance measured alongthe longitudinal axis of said cylindrical wave guide from said endclosure to the midpoint of said aperture projected perpendicularly tosaid axis being approximately .835 of said wave length.

9. In a mode transformer of the type claimed in claim 8, wherein aninductive iris is positioned within said rectangular wave guide andspaced from the juncture of said rectangular wave guide and saidcylindrical wave guide by a distance of approximately .144 of the freespace wave length of the energy being propagated.

References Cited in the file of this patent UNITED STATES PATENTS2,267,289 Roosenstein Dec. 23, 1941 2,407,318 Mieher Sept. 10, 19462,427,100 Kihn Sept. 9, 1947 2,432,093 Fox Dec. 9, 1947 2,433,011Zaleski Dec. 23, 1947

